Ophthalmic lenses, in particular contact lenses, which it is intended to produce economically in large numbers, are preferably produced by the so-called mold or full-mold process. In this process, the lenses are produced in their final shape between two mold halves, so that neither subsequent machining of the surfaces of the lenses nor machining of the edge is necessary. Such molding processes are described in more detail, for example in WO-A-87/04390, EP-A-0367513 or in U.S. Pat. No. 5,894,002.
In the known molding processes, the geometry of the ophthalmic lens to be produced is defined by the mold cavity between the two lens forming surfaces of the mold halves. The edge of the ophthalmic lens is likewise formed within the mold, whereby the geometry of the edge may be defined by a spatial limitation of the UV light used for cross-linking a lens forming material.
In order to produce an ophthalmic lens, such as a contact lens, usually a specific amount of a flowable lens forming material is introduced into the female or concave mold half in a first step. The mold is then closed by putting the male or convex mold half into place. The subsequent polymerization and/or cross-linking of the lens forming material is carried out by means of irradiation with UV light and/or by heating. In the process, either both, the lens forming material in the mold cavity and the excess material in an overflow region or area are crosslinked or hardened, or only the lens forming material in the mold cavity is hardened, whereas the excess material in the overflow region remains as “flash”. In order to obtain fault-free separation of the lens from the excess material, good sealing or expulsion of the excess material must be achieved in the zone in which the two mold halves make contact with each other or in that zone, which defines the spatial limitation of the UV light used for cross-linking the lens forming material.
After the lens has been formed, the mold is disassembled and the lens removed. Additional processing steps, such as inspection, extraction, hydration, surface treatment and sterilization may finally be performed on the lens before packaging.
Upon polymerization and/or cross-linking of the lens forming material, shrinkage may occur, which may lead to stress in the final lens and/or may result in an uneven lens surface which may be referred to as having so-called “grooves” or “road maps”. The problem of shrinkage upon curing (i.e. polymerization and/or cross-linking) of lens forming material is known in the art and has been addressed with different technical solutions, such as the following:
EP-A-1014 155 discloses a method of manufacturing a contact lens having areas of different thickness, wherein the contact lens design is compensating differential shrinkage during the manufacturing process. The contact lens design is providing an equal but opposite power to compensate for unwanted toricity induced by the manufacturing process.
U.S. Pat. No. 4,197,266 discloses an apparatus for forming ophthalmic lenses, the apparatus comprising cooperating first and second mold surfaces to form a mold cavity. The assembled molds further comprise an annular reservoir connected with the mold cavity. In operation said reservoir provides lens forming material to the mold cavity of the lens, to compensate for shrinkage of the lens material upon curing.
U.S. Pat. No. 5,269,867 discloses a method for producing optical devices, wherein curing is effected by irradiation of the resin through a filter so as to be uniformly incident on the resin, thereby preventing stress or shrinkage on the resin surface upon curing.
In spite of all prior art attempts shrinkage is still a problem to be addressed in the manufacture of ophthalmic lenses, in particular in the fully automated manufacture of contact lenses using rigid molds, and even more so in the manufacture of silicone hydrogel contact lenses. The present invention therefore is directed to an improved process and apparatus for the manufacture of an ophthalmic lens, in particular a silicone hydrogel contact lens.
It is an object of the present invention to provide an improved process for manufacturing an ophthalmic lens, in particular a silicone hydrogel contact lens, which reduces or avoids defects, in particular uneven lens surfaces (with so called “grooves” or “road maps”), which may occur due to shrinkage of the lens forming material upon polymerization and/or cross-linking.
It is a further object of the present invention to provide an improved apparatus for forming an ophthalmic lens, in particular a silicone hydrogel contact lens, which reduces or avoids defects, in particular uneven lens surfaces (with so called “grooves” or “road maps”), which may occur due to shrinkage of the lens forming material upon polymerization and/or cross-linking.
It is still a further object of the present invention to provide an improved process and apparatus for manufacturing an ophthalmic lens, in particular a silicone hydrogel contact lens, which reduces stress in the final lens that may occur due to shrinkage of the lens forming material upon polymerization and/or cross-linking.